Positive and negative‐mode laser desorption/ionization‐mass spectrometry (LDI‐MS) for the detection of indigoids in archaeological purple

Abstract

Laser-based ionization techniques have demonstrated to be a valuable analytical tool to study organic pigments by mass spectrometric analyses. Though laser-based ionization techniques have identified several natural and synthetic organic dyes and pigments, they have never been used in the characterization of purple. In this work, positive and negative-mode laser desorption/ionization mass spectrometry (LDI-MS) was used for the first time to detect indigoids in shellfish purple. The method was used to study organic residues collected from archaeological ceramic fragments that were known to contain purple, as determined by a classical high-performance liquid chromatography-based procedure. LDI-MS provides a mass spectral fingerprint of shellfish purple, and it was found to be a rapid and successful tool for the identification of purple. In addition, a comparison between positive and negative mode ionization highlighted the complementarity of the two ionization modes. On the one hand, the negative-ion mode LDI-MS showed a better selectivity and sensitivity to brominated molecules, such as 6,6'-dibromoindigo, 6-monobromoindigo, 6,6'-dibromoindirubin, 6- and 6'-monobromoindirubin, thanks to their electronegativity, and produced simpler mass spectra. On the other hand, negative-ion mode LDI-MS was found to have a lower sensitivity to non-brominated compounds, such as indigo and indirubin, whose presence can be established in any case by collecting the complementary positive-ion LDI mass spectrum.

Towards matrix‐free femtosecond‐laser desorption mass spectrometry for in situ space research

“…[30] Furthermore, direct LD studies of organic molecules have successfully been carried out without the assistance of a matrix or substrate pretreatment. [3,11,[31][32][33][34][35] These investigations are particularly encouraging for in situ space research where minimal or no sample preparation procedures and no spatial disruption of the analyte prior to the analysis are of prime importance. [36][37][38] In this context, special attention is currently being devoted to the quest for low molecular weight biomarkers of extinct or extant life on planetary objects beyond Earth.…”

“…Furthermore, direct LD studies of organic molecules have successfully been carried out without the assistance of a matrix or substrate pretreatment . These investigations are particularly encouraging for in situ space research where minimal or no sample preparation procedures and no spatial disruption of the analyte prior to the analysis are of prime importance .…”

“…The analysis of organic species in solid materials by mass spectrometry (MS) has become increasingly important in various fields of research, including developments in biomaterials, mineralogy, archaeology, astrobiology, electronics, metallomics, molecular imaging, and tribochemistry . In addition to Thermal Desorption Photoionization MS (TDPI‐MS), Glow Discharge MS (GD‐MS), Laser Ablation Inductively Coupled Plasma MS (LA‐ICP‐MS), and Secondary Ion MS (SIMS), analytical methods based on soft Laser Desorption/Ionization MS (LD‐MS) are at the forefront of these developing technologies.…”

Characterization of natural organic colorants in historical and art objects by high‐performance liquid chromatography

“…Since that time, several new papers dealing with LC–MS of flavonoid glucuronides from sawwort , plant‐derived flavonol conjugates , newly elucidated anthraquinoid components of cochineal , and, as mentioned above, recently discovered indirubin analogues from shellfish purple were published. Simple MS methods without chromatographic separation and using direct ESI or laser desorption–ionization MS can confirm the presence of a certain dyestuff.…”

Identification of collagen-based materials in cultural heritage

“…[5][6][7][8][9] Immunological techniques are being developed to identify proteins and gums in artworks; 10 and laser desorption mass spectrometry (LDMS) is being used to complement traditional methods for identifying organic dyes and pigments in paintings and sculpture. [11][12][13][14][15][16] The increasing availability of analytical tools and techniques provides an exceptional opportunity for conservators to acquire technical information about art and cultural heritage heretofore unobtainable. It is in this vein that we have adapted PMF as a routine means of identifying collagen-based materials in a museum laboratory.…”